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Sound and Wave Phenomena
Published in David M. Scott, Industrial Process Sensors, 2018
Electromagnetic speakers, commonly found in radios, consist of a small coil of wire (called a voice coil) that is connected to a paper cone or diaphragm. When electrical current passes through the voice coil, it creates a fluctuating magnetic field within the coil. An external magnetic field is generated by a nearby permanent magnet, and the interaction between the two fields produces a force on the coil. This force is transmitted to the diaphragm, which pushes on the surrounding air. Most speakers can be used as microphones, since sound waves can push on the diaphragm and move the voice coil within the external magnetic field. If the field is not uniform, then the changing amount of magnetic flux through the voice coil causes a small voltage to appear across the coil.
Permanent-Magnet Direct-Current Motion Devices and Actuators
Published in Sergey Edward Lyshevski, Mechatronics and Control of Electromechanical Systems, 2017
A lightweight cone- or dome-shape diaphragm is connected to a rigid frame using a flexible suspension. A variety of different materials are used, such as coated paper, plastic, and composite materials. An N-turn winding (voice coil) is under the stationary magnetic field established by radially magnetized permanent magnets, as illustrated in Figure 4.18a. To displace a diaphragm, one applies the voltage to the winding. The electromagnetic force F→=∮lidl→×B→=−i∮lB→×dl→ is produced. The suspension system maintains the coil centered within the gap and provides a restoring elastic force to make the cone return to an equilibrium position if voltage is not applied. Insulated copper and silver wire is used to fabricate a voice coil within a circular, rectangular, or hexagonal cross section. The coil is oriented coaxially inside the gap. Ceramic, ferrite, alnico, and rear-earth magnets are used.
What is a Loudspeaker?
Published in Douglas Self, Audio Engineering Explained, 2012
To a casual observer, a typical moving-coil loudspeaker (or “driver” if you wish to restrict the use of “loudspeaker” to an entire system) seems to be a simple enough device. There is a wire “voice-coil” in a magnetic field. The coil is wound on a cylindrical former which is connected to a cardboard cone, and the whole thing is held together by a metal or plastic chassis. The varying electrical input gives rise to vibrations in the cone as the electromagnetic field in the voice-coil interacts with the static field of the (usually) permanent magnet. The cone thus responds to the electrical input, and there you have it, sound! It is all as simple as that! Or is it?
Dielectric properties of the chitosan – spinel CoFe2O4 composites
Published in Phase Transitions, 2018
E. Markiewicz, K. Chybczyńska, A. Grząbka-Zasadzińska, S. Borysiak, J. Wolak, M. Matczak
Composites are the materials consisting of several components interconnected by physical forces (e.g. adhesion). The properties of composites differ from those of its individual components. The proper choice of components and the technology of their interconnection permit production of composite materials of properties desired for specific applications. Some composites have been known for several thousand years, e.g. the Ramesses army (c. 1327–1237 BCE) in ancient Egypt was equipped with the composite bows prepared from properly chosen plant fibers and glues. Because of the outstanding elastic properties these bows had considerably longer range in comparison with those used by the enemies (e.g. Hittites in the bottle of Kadesh, 1302 BCE) which were made of homogenous wooden bars. Nowadays, worldwide laboratories tend to search for environmentally friendly composites with distinctive dielectric and magnetic properties suitable for application in modern transducers, for instance a loudspeaker without a permanent magnet [1]. The traditional loudspeaker has a voice coil, fixed to the membrane, which is suspended in the magnetic field of a permanent magnet. The current applied to the coil drives the coil-membrane system to move in the air generating the acoustic wave. In the proposed loudspeaker prototype [1] a composite membrane plays the role of the permanent magnet resulting in a considerable reduction of dimensions and mass of the loudspeaker.